And how do you plan to get up to that speed? If you’re going to use something with motorised assistance, it will almost certainly be a device that the State will class as a road vehicle. Driving on the sidewalk is illegal.
Any "limit in practice" is not a prescribed limit. The whole point of posted, visible speed limits is that everyone knows what they are ahead of time. They're not a substitute for using your brain.
(Also in practice if you go too fast you'll run up against physics and literally burn up due to atmospheric drag.)
> And how do you plan to get up to that speed?
OP's example is a world-class track sprinter. Usain Bolt just runs fast (for 100-400m).
In my state, non-motorized bicycles may use the sidewalk freely. When riding on the sidewalk, cyclists are considered pedestrians. They (like all other pedestrians) must be courteous to other pedestrians, but there is no speed limit.
In theory, on a long enough sidewalk without ramps, alleys, obstructions, etc, a cyclist could plausibly exceed 30 mph without being discourteous to anyone or endangering others. There are some sidewalks like this in the industrial parts of my town, although I'm certainly not suggesting anyone try it.
I don't understand the point of stating the blindingly obvious as though it could serve as a useful retort. Obviously there isn't a prescribed limit. That doesn't elide the point—a limit doesn't need to be a bright line in order to exist in practice.
My argument is wrong because it is silly. Gotcha.
>do you also want to ban usain bolt from the sidewalk
When Usain Bolt can delete a family from existence with one wrong flick of the wrist, I'll support banning Usain Bolt from running in the sidewalk.
Alternatively, when Usain Bolts running in sidewalks are one of the leading worldwide causes of death.
And I'm pretty sure that if Usain Bolt were running at 30kph on a crowded sidewalk somebody would stop him, which is actually a pretty nice analogy to the obviously beneficial measure of placing a speed limiter on cars.
No, it's so wrong that it is silly. That wasn't supposed to be the reason why.
Stop looking for cheap gotchas.
do you also want to ban usain bolt from the sidewalk because he could run too fast if he wanted to?